The present invention relates generally to a method of preparing thermally conductive interface materials and compounds for improving heat transfer from a heat generating semiconductor device to a heat dissipator device such as a heat sink or heat spreader. More specifically, the present invention relates to a method and/or technique for preparing a mixture of an indium alloy blended with a polymer matrix, the polymer being in the solid phase at room temperatures and with both the alloy and the polymer having a melting temperature of between about 40xc2x0 C. and 120xc2x0 C., preferably between about 40xc2x0 C. and 100xc2x0 C. These blends of metal alloy and polymer have been found to sharply reduce the thermal resistance or impedance which typically arises from a less-than-perfect contact between the boundaries or surfaces of a thermal interface positioned between the components of the assembly. More particularly, the present invention involves a process for blending a normally solid polymeric matrix with a low melting alloy of indium metal for forming an improved thermal management system for use in combination with high performance semiconductor devices.
The thermal impedance or resistance created between two components in a typical electronic thermal management assembly is increased when surface imperfections are present on the opposed surfaces of the two components. The causes of poor physical contact typically lie with macroscopic warpage of one or both surfaces, surface roughness, or other non-flat characteristics created on one or both of the opposed contact surfaces. Areas of non-intimate surface contact result in the creation of air-filled voids which are, of course, exceptionally poor conductors of heat. High thermal impedance resulting from poor thermal contact results in undesirable heating of electronic components which in turn accelerates the rate of failure of the components such as semiconductor components and comprising the assembly. Replacement of air gaps or voids with a thermally conducting medium comprising a good thermal management system has been found to sharply reduce the thermal impedance and/or resistance.
In the past, liquid metals have been proposed for incorporation in thermally conductive pastes for use with heat generating semiconductor devices. In some cases, liquid metals were not readily adapted for this purpose, primarily because of problems created with the tendency of the liquid metal to form alloys and/or amalgams, which altered or modified the thermal and other physical properties of the mounting systems. Other thermal interface materials are made by dispersing thermally conductive fillers in a polymer matrix. While most polymer matrices range in thermal conductivity from 0.1-0.2 W-mxe2x88x921-Kxe2x88x92, the properties of the fillers are quite varied. They include silica (2 W-mxe2x88x921-Kxe2x88x92), zinc oxide (10-20 W-mxe2x88x921-Kxe2x88x92), alumina (20-30 W-m-1xe2x88x92Kxe2x88x92), aluminum nitride (100 W-m-1xe2x88x92Kxe2x88x92), and boron nitride (200 W-mxe2x88x921-Kxe2x88x92). When placed in the thermal joint, these compounds are intended to displace air and reduce overall thermal impedance. Addition of thermally conductive fillers, generally consisting of fine particulates, improved the thermal conductivity of the compound filling the voids.
In our copending application Ser. No. 09/543,661, a number of low melting alloys are disclosed which are highly effective for use as thermal interfaces in thermal management systems for enhancement of percolation of thermal energy. The present invention provides additional advantages in thermal interfaces through the use of certain selected polymer matrices for retention of the low melting alloy, the matrices having melting points which are also low and, preferably, relatively close to the melting points of the retained alloys. These polymers as well as the alloys are in solid phase at room temperature, and this feature facilitates ease of handling of the thermal interface particularly during production and use.
In accordance with the present invention, improved interface materials have been developed based on incorporation of low melting alloys as fillers capable of altering their shape in response to heat and pressure. At room temperature, these fillers are in solid phase, as is the polymer matrix, with this combination of features facilitating ease of handling. In addition, these morphing fillers respond to heat and pressure by their ability to flow into and fill air gaps or voids that may be present in the matrix, thereby avoiding creation of standoff or poor particle-to-particle contact (see FIG. 2).
In those applications where the opposed surface areas are small, or alternatively are relatively flat, interfaces having thin cross-sections may be employed. Typically, in such applications, those dispersions utilizing only polymeric matrices having dispersed low melting alloys function well (see FIG. 3). For interfaces employing a laterally disposed mechanical standoff, or those subject to large warpage, it is normally desirable to utilize highly thermally conductive particulate fillers in combination with the low melting alloys in order to create large heat percolating clusters (see for example FIG. 4).
In accordance with the present invention, an indium-containing alloy is selected which is in the solid phase at room temperature, while having a melt temperature of between about 40xc2x0 C. and 120xc2x0 C. The alloy is then subjected to a size reduction operationxe2x80x94typically by emulsifying, while in molten phase, in the polymer matrix of interest. A surface active agent may be added during the emulsification to enhance the rheological properties and dispersion stability. Alternatively, the size reduction of the metal alloy may be accomplished by blow or impact, or alternatively by grinding or abrasion, under cryogenic conditions. Depending upon the particular type of equipment and conditions under which the particulate is formed, it may be possible to add the surface active agent to the working material while undergoing size reduction process. The metallic powder can then be blended with a quantity of a matrix polymer which is likewise in the solid phase at room temperature, having a melt point of between about 40xc2x0 and 100xc2x0 C. to form a compliant pad. The polymer matrix is preferably selected from the group consisting of paraffin, microwax, and silicone waxes. The low melting alloy may also be blended with a particulate filler such as, for example, boron nitride or alumina with the resultant mixture being mechanically agitated in the presence of a compatible wetting agent to form a stable dispersion for ultimate blending with the polymer matrix.
It should be noted that while the melt temperatures for the polymer matrix and the metal alloy are both indicated as being between about 40xc2x0 C. and 120xc2x0 C., it is desirable that a differential be maintained between the actual melt temperatures. For example, it has been found desirable to select a polymer matrix having a melting temperature which is approximately 10xc2x0 C. lower than that of the metal alloy. Other differential relationships may also be useful. While certain other metal alloys may be found useful, indium-based alloys are generally preferred for utilization in the present invention.
The physical properties of thermal interface compounds prepared in accordance with the present invention are such that conventional production handling techniques may be employed during assembly operations. In this connection, the compounds may be handled or formed into an interface device by stamping or they may be printed directly onto heat-transfer surfaces. Alternatively, they may be made into tapes that can be die-cut so as to be later applied directly onto the heat transfer surfaces.
Therefore, it is a primary object of the present invention to provide compositions of materials useful as thermal interface compounds, wherein a low melting metallic alloy is retained within a polymer matrix, and wherein each of these components is in the solid phase at room temperature, and has a melting temperature of between about 40xc2x0 C. and 120xc2x0 C. and preferably between about 40xc2x0 C. and 100xc2x0 C.
It is a further object of the present invention to provide an improved combination of components utilized to form a composition which is useful as thermal interface compounds, and wherein hard particulate fillers such as boron nitride and/or alumina may be employed in combination with an indium alloy, and thereafter blended into and retained within a polymeric matrix.
It is yet a further object of the present invention to provide an improved thermal interface compound which is dry and solid at room temperature, and which changes to liquid phase at moderately elevated temperatures, thereby permitting the compounds to be easily handled utilizing conventional handling techniques and yet respond effectively in a thermal management application.
Other and further objects of the present invention will become apparent to those skilled in the art upon a study of the following specification, appended claims, and accompanying drawings.